I have read a lot of post about how long a rooster's sperm last in a hen and the general consensus is three weeks. But I have a different question. My question is this, if a hen is inseminated by one rooster and a week later by another rooster; assuming both roosters sperm are active, which roosters sperm will fertilize the egg? I mean what happens when a hen has active sperm from multiple roosters?

Well, as long as there is a chance the right rooster is the father, it okay with me. I had my three Barred Cochin Bantam hens penned up with a rooster for a couple of weeks, but they got out into the general population for a few hours and I don't know it they were mated by my other Heinz 57 roosters. A friend of mine wants some eggs from them to incubate. There is always someone who will take a mix breed if some of the eggs are mixed.

I have never seen my full size roosters mate with my Bantams. I wonder it they can?

I just hatched 19 white rocks. The first thing I did was to lock the girls up alone for three weeks by themselves in a coop and run that is visit prove. The run is 7 ft tall and covered. After three weeks I put the wr roo in with them and waited 3 days before saving eggs, and then saved 21 eggs for the bator. 1 was clear and was pulled at day 10 and 20 went into lockdown . 19 hatched and are doing well at 3 weeks, but 2 of them are black and have markings like the barred rock roo the girls used to range with.
This has taught me that if I want pure breed chicks to wait 4 weeks to be sure because 3 may not be enough.

If you want to make sure a hen is fertilized by only one rooster, let her go broody and set for awhile. This will kill off any sperm she has in her.
Break her up and put her with the selected rooster. She may not lay for a bit but her next eggs will be only fertilized by him. Tried it myself and it does work. Only thing is it may be time consuming, but it's definite.